Some polymer networks take up water and swell. These swollen hydrogels have been created from a variety of starting materials and have been used for a variety of applications. The utility of prior hydrogels for their proposed applications is limited by the properties of these compositions, however. In addition, the starting materials and processes of making and using such prior compositions limit not only the resulting properties of the polymers but also the commercial viability of the manufacturing processes and articles made in such processes. Also, the mechanical properties of the prior polymers are often limited by the mechanical properties of the component polymers used, which in the case of most intrinsically hydrophilic, water-swellable polymers, are usually quite low.
Hydrogels have been combined with polyurethanes to form articles with certain useful properties. Hydrogel materials have been reinforced with polyurethanes and other materials to provide a more robust backing material. Also, a hydrogel coating or overlayer can be added to a hydrophobic polymer article to improve the article's biocompatibility. Prior hydrogel/polyurethane combinations have not provided the best combination of strength and swellability, however. In addition, prior methods of making hydrogel/polyurethane combinations have used expensive and/or toxic processes.
For example, Hoffman et al. U.S. Pat. No. 3,826,678 describes a process for coating an inert polymeric substrate with a reactable hydrogel polymer and then attaching a biologically active molecule in order to make a biocompatible material with a biofunctional surface. Hoffman used “radiation grafting” to attach the hydrogel to the polyurethane and used the reactable hydrogel to attach the biologically active molecules. Hoffman's “radiation grafting” refers to application of expensive high energy to a polymer, a treatment that causes both non-specific formation of bonds and non-specific breaking of bonds. The bonds made are non-specific bonds between the two polymers anywhere along the backbone of the chains, as well as non-specific bonds (“crosslinking”) within each polymer. Conditions for using “radiation grafting” were chosen by Hoffman such that more favorable than unfavorable reactions occur.
Hoffman used “radiation grafting” on a preformed material, such as a polyurethane, that had been contacted with a preformed hydrogel or with hydrogel monomers, and then subjected the materials to the high energy radiation (e.g., gamma irradiation or X-rays). The result was crosslinked hydrogel pieces non-specifically grafted to crosslinked polyurethane pieces. Next, the biologically active materials were attached using a specific link that bonded the biomaterial to the hydrogel. In this way, the biological materials were never subject to the fragmentation effects of the radiation treatment, and a bioactive material was made.
Yang et al. (J. Biomed Mater Res 45:133-139, 1999) describe a process for forming a graft material having both polyurethane and hydrogel. Yang formed a mixture of polyurethane, acrylic acid, and photoinitiator, and treated it with UV light in the absence of a degassing step to create an homogeneous and unlayered acrylic acid/polyurethane polymer grafted throughout its composition.
Park and Nho (Radiation Physics and Chemistry; 67 (2003): 361-365) describe making a wound dressing formed from polyurethane and hydrogel layers. First, polyurethane was dissolved in solvent and dried to form a polyurethane layer. Then a mixture of polyvinyl alcohol/poly-N-vinylpyrrolidone, chitosan and glycerin in water was poured onto the already formed polyurethane layer. The material was optionally treated with freeze-thaw cycles. Conditions were chosen to favor cross-linking reactions in the hydrogel over material degradation during irradiation treatment, and the material was subject to gamma irradiation to form a hydrogel. The result was a hydrogel adjacent a polyurethane; Park et al. do not describe the nature of any interaction between the polyurethane layer and the hydrogel layer.
Wang et al. (U.S. Patent Publication 2002/00524480) describe a process for forming a material having a modified surface that can be used to tether other compounds at the surface while maintaining the bulk properties of the material. Wang started with a foamed hydrophobic polymer, such as an acylic or polyurethane, and introduced a functional monomer such as acylate or vinyl monomer, and an initiator just at the polymer surface, such as by limited swelling of the polymer in a solvent. The functional monomer was treated, such as with UV irradiation, to form a second polymer. A surface modification agent, such as heparin, may be attached to the second polymer. The result was an Interpenetrating Polymer Network (IPN) at the surface between the polymerized formed polymer, with only indirect interactions between the first and second polymers, and possibly modified with a modification agent covalently attached to the second polymer.
Gao et al. (Chinese Journal of Polymer Science Vol. 19, No. 5, (2001), 493-498) describe improvements to materials for use in improving long-term implants that become integrated into the body, such as devices put into blood vessels and in artificial hearts. Gao describes two methods to create on a segmented polyurethane a hydrophilic surface containing functional groups that will adhere cells and support growth. In both methods, the segmented polyurethane was activated by a high concentration of toxic hydrogen peroxide (30%) and UV light to generate reactive groups.
In the “Solution Grafting Method,” of Gao, the activated segmented polyurethane was immersed in a solution of hydrophilic monomers, such as 2-(dimethylamino)ethyl methacrylate, 2-hydroxyethyl acrylate or acrylamide, and ammonium iron (II) sulfate hexahydrate, and the monomers grafted onto the segmented polyurethane by treatment with UV light. The iron compound prevents any unwanted polymerization of the monomers in solution.
In the “Pre-Absorbing Grafting Method” of Gao, the activated segmented polyurethane membrane was immersed in a solution of hydrophilic monomers, removed, placed under nitrogen, and the hydrophilic monomers grafted onto the reactive groups of the segmented polyurethane by treatment with UV light. The membrane was rinsed with hot water for 48 hours to remove homopolymers. The result was a very thin layer of hydrophilic polymer coating on the surface of the polyurethane. SEM images of materials made using the “Solution Grafting Method” versus those made using “Pre-Absorbing Grafting Method” show significant differences in appearance in materials made using the different methods.